Polyesters are very interesting materials because of the properties that these materials can exhibit. These properties, for instance, include biocompatibility, biodegradability and drug permeability. Therefore, polyesters are of great interest for medical and food packaging applications. For these purposes materials with an engineered structure are desired, which implies the need for a high level of control over the polymerization reaction. In addition, with the right properties, polyesters can form an interesting biodegradable alternative for polyethylene in many applications. Traditional polyester synthesis strategies, using e.g. polycondensation, give rise to fundamental problems that can make the controlled synthesis of these materials a tedious process. For example, the preparation of polyesters by polycondensation can be accompanied by stoichiometric problems, the need for high conversion and the removal of small molecules formed during the reaction.
A suitable replacement for these conventional strategies is the ring-opening polymerization of lactones. This polymerization is based on the fact that cyclic monomers “open up” and form a polymer chain by means of a chain-growth process. However, ring-opening polymerization reactions can also be difficult to control, in particular when anionic or cationic initiators are used.
It is known that ring-opening polymerization reactions can be performed with enzymes with satisfactory conversion under mild polymerization conditions. For example, lipases such as Candida Antarctica Lipase B (CALB) are highly active in the ring-opening polymerization of lactones and show exceptionally high polymerization rates for macrolactones. The reactivity of lactones in this process is not governed by the high ring-strain of small lactones (cisoid ester bonds) but by the preference of the lipase for transoid ester bond conformation present in large ring lactones. Macrolactones can thus easily be polymerized by CALB. For example, poly(pentadecalactone) with a number average molecular weight up to 150 000 g/mol have been reported (Focarete et al., J. Polym. Sci. B: Polym. Phys. 2001, 39, 1721 and De Geus et al., Polym. Chem. 2010, 1, 525).
However, control over molecular weight and polydispersity index (in particular a polydispersity index of ≧2) of the resulting polyester is limited. Moreover, ring-opening polymerization with enzymes is strongly limited by the applied temperature, because enzymes will typically not withstand higher reaction temperatures. In addition, the enzymes that can be used for ring-opening polymerization of lactones are rather expensive.
In view of the limitations of enzymatic ring-opening polymerization, attempts have been made to find suitable alternative metal-mediated ring-opening polymerization processes. Such processes are particularly attractive, because they allow a high level of control over the polymer molecular weight, the molecular weight distribution, copolymer composition and topology and end-groups by using a nucleophilic initiator. It is commonly agreed that the driving force behind the ring-opening polymerization of lactones is the release of ring-strain in the transition from the cyclic ester to the polyester chain or, in thermodynamic terms, by the negative change of enthalpy. Consequently, as the ring-strain decreases with increasing lactone size so does the reactivity in metal-mediated ring-opening polymerization. Experimentally, this was shown by Duda in a comparative study of the ring-opening polymerization of various size lactones using zinc octoate/butyl alcohol as a catalyst/initiator (Duda et al., Macromolecules 2002, 35, 4266). While the relative rates of polymerization were found to be 2500 and 330 for the six-membered (δ-valerolactone) and seven-membered (ε-caprolactone) lactones, respectively, the reaction rates of the 12-17 membered lactones were only around 1. Consequently, only a few examples of metal-catalyzed ring-opening polymerization of macrolactones like 15-pentadecalactone can be found in literature, while those examples that can be found only report low yields and low molecular weights. The best results were obtained using yttrium tris(isopropoxide) leading to acceptable conversions and molecular weights of up to an absolute number average molecular weight of 30 000 g/mol (Zhong et al., Macromol. Chem. Phys. 2000, 201, 1329).
WO 2006/108829 relates to a method for producing polyhydroxyalkanoates by the polymerisation of lactones in the presence of at least one catalyst of formula L1MaXam.
JP 2001/0255190 discloses a lactone ring-opening polymerization catalyst which can simply produce stereocomplexes having sufficiently high thermal stability. This lactone ring-opening polymerization catalyst contains a salen type metal complex and is useful for producing a polyester and a block copolymer, in particular for producing biodegradable plastics and mendical materials.
WO 2010/110460 discloses a method for producing a lactide/epsilon-caprolactone copolymer whereby a lactide/epsilon-caprolactone copolymer being close to an ideal random copolymer can be produced while controlling the molecular weight and the molecular weight distribution. Lactide is copolymerized with epsilon-caprolactone by using an aluminum-salen complex as a catalyst.
The scientific article “Ring opening oligomerisation reactions using aluminium complexes of Schiff's bases as initiators” (Le Borgne et al., Makromol. Chem., macromol. Symp. 73, 37-46 (1993)) discloses aluminium initiators derived from Schiff's bases which were successfully used for oligomerization of oxiranes, lactones and lactides.
In view of the prior art, it would be highly desirable to provide a suitable catalyst for metal-mediated ring-opening polymerization of lactones capable of achieving similar conversions and molecular weights as reported for enzymatic ring-opening polymerization. Furthermore, it would be desirable to combine the advantages of enzymatic ring-opening polymerization of lactones with the thermostability of metal-mediated ring-opening polymerization and with the versatility of metal-mediated ring-opening polymerization regarding control of molecular weight, molecular weight distribution and end-groups.